1. Field of the Invention
The present invention relates generally to microwave isolation switches and more particularly to such microwave isolation switches utilizing strip transmission lines.
2. Description of the Prior Art
Many radar systems are designed to operate either in a linear polarization mode or a circular polarization mode. While the normal mode of operation is the linear mode under some weather conditions such as heavy rain there is considerable advantage in using circular polarization. This results from the fact that rain drops which are spherical reflects circularly polarized waves back as circularly polarized waves that are polarized in the opposite sense whereas most targets are non-spherical and reflect but a portion of the circularly polarized transmitted waves. The reflected portion being elliptically or linearly polarized.
In addition to having a selection of polarization modes for weather condition the plurality of modes are also useful in combat operations as the radar is less susceptible to jamming. It has been proven in actual flight tests that anti-jamming performance can be enhanced by being able to change to a left-hand circular polarization as well as the commonly used right-hand circular and vertical linear polarizations.
In one heretofore known device for selectively switching modes a quarter-wave plate is rotatably mounted within a feed horn that can be rotated in a clockwise or counter-clockwise direction. With the feed horn being rotated in a counter-clockwise direction friction between the outer race of a bearing mounting of quarter-wave plate and the inner race causes the quarter-wave plate to be rotated until its housing is stopped by a mechanical stop. In this position which is referred to as the left-hand circular mode, the quarter-wave plate is positioned diagonally at approximately 45.degree. to the vertical axis of the feed horn. In order to change to a right-hand circular mode, it is merely necessary to change the direction of the rotation of the feed horn. With the feed horn rotating in a clockwise direction the quarter-wave plate will be rotated 90 degrees until its housing encounters a second mechanical stop. This position is referred to as the right-hand circular mode. In order to change from a left-hand circular mode to a linear polarization mode, an electromagnet is energized which in turn causes a third stop to be pivoted into position such that it will permit the quarter-wave plate to rotate only 45.degree.. The direction of rotation of the feed horn is then changed to a clockwise direction and the quarter-wave plate is rotated in that direction until its housing is stopped by the third mechanical stop that was previously pivoted into position.
In one heretofore known device for selectively switching mode, an antenna feed horn is continuously rotated in one direction and each of three polarizations, i.e., left-hand circular, right-hand circular or linear, can be selected by energizing certain electromagnets. A quarter-wave plate is mounted within a housing that is provided with three catch surfaces, with the housing being rotatably mounted with respect to a rotating feed horn. Three separate levers, which are actuated by electromagnets, are pivotally mounted in a stationary ring within the feed horn and by selectively energizing the electromagnets various levers can be made to engage various catch surfaces. When no lever is engaged with any catch surface, the housing, and consequently the quarter-wave plate, is rotated due to the friction of the bearing that mounts the housing to the rotating feed horn.
While the above described methods of changing polarizations do work satisfactorily there are several inherent disadvantages in these methods of polarization selection. First, the time lag involved in reversing feed horn motor could cause the radar to lose its lock-on status. Also errors result in the positioning of the reference generator when the direction of the feed horn is reversed since the generator is nulled for either clockwise or counter-clockwise rotation. In addition, such polarization devices are custom designed for a particular antennas feed thus requiring time consuming and inconvenient adaptations for use with other antennas. Such polarization devices are also large and bulky as well as being heavy. In addition, they can be produced only at high costs.
Other major problems include phase and amplitude inbalance occurring when employing coaxial cables and connectors for interconnecting the hardware. Often an additional coaxial phase shifter is required.